Carbon sequestration is a viable alternative to reduce the emissions of the greenhouse gas carbon dioxide (CO2) from large point sources. It holds the potential to provide deep reductions in greenhouse gas emissions. Carbon sequestration is a two-step process where the capture of carbon dioxide from a gas stream is followed by permanent storage. The capture step for carbon dioxide represents a major cost in the overall process.
Of particular interest are power generation point sources that use fossil fuels. Since nearly one-third of the anthropogenic CO2 emissions are produced by these facilities, conventional coal-burning power plants and advanced power generation plants—such as integrated gasification combined cycle—present opportunities where carbon can be removed and then permanently stored. At the current time, pulverized coal-fired-base steam cycles have been the predominant electric power generation technology. These will continue to be used predominantly in the near future. Technologies for capturing CO2 will need to be applied to new more efficient coal-fired facilities and will need to be retrofitted onto existing plants.
Effective amine-based solid sorbent methodologies are needed for carbon dioxide capture from a gaseous mixture, whether the capture occurs in combustion or gasification power generation systems from flue gas, or in other applications such as natural gas sweetening. Because of the high concentration of carbon dioxide in any of these feed streams, a large quantity of the gas will react with the sorbent and thus produce considerable amounts of exothermic heat. This heat must be removed from the sorbent to prevent temperature instability within the reactor, to assure the sorbent will operate at optimum temperature, and to eliminate the potential degradation of the sorbent because of high temperature excursions.
For coal-fired power plants, the conventional scrubbing system that is currently the comparative baseline for all other capture technologies is monoethanolamine (MEA) scrubbing. This wet scrubbing process removes the CO2 in an absorber and then regenerates the spent scrubbing liquor in a vessel by indirectly heating the solution with plant steam. Although there have been large scale commercial demonstrations of this technology, the process has several disadvantages, such as a high heat of reaction, low working capacity, corrosiveness of the solution, the susceptibility of being poisoned, and most notably, its need to be in an aqueous solution. This latter disadvantage results in a large energy need to regenerate the spent solution, especially the sensible heating of the water, which is a minimum of 70 wt % of the solution. The water is recognized as an inert carrier between the absorption and regeneration steps. Another energy loss while regenerating the spent MEA solution includes evaporative heat loss of vaporizing liquid water.
One type of novel CO2 capture technology that can be applied to various gas streams has, as a basis, dry regenerable solid sorbents. Examples of these types of sorbents are zeolites, activated carbon, alkali/alkaline earth metals, immobilized amines, metal organic framework, etc. A specific sorbent category that shows significant advancement are amine-based solid sorbents, such as Basic Immobilized Amine Sorbents (hereinafter BIAS). BIAS consist of amines (primary, secondary, tertiary, or a combination thereof) deposited onto a porous support. The manner of deposition can be random or a structured deposition of the amine onto this support (silica, polymer, etc.). When used in the industrial setting, the dry solid sorbent process may act in a similar fashion to the wet scrubbing process in that the sorbent would be transported between an adsorption step and a regeneration step and in that the sorbent is regenerated by a temperature-swing application.
One of the main benefits in using the solid sorbent is the elimination of the sensible heat for the liquid water as compared to MEA. A secondary benefit lies in the lower heat capacity for the solid versus the liquid solvent, also serving to lower the sensible heat required. More CO2 can be adsorbed on a weight or volume basis with the amine-based solid sorbents, so the sorbent system is capable of a significant decrease in the heat duty for the regeneration step. A lower cost of energy service for process involving BIAS as compared to amine wet scrubbing may also result. Thus amine-based solid sorbents have the capability to improve the overall energetics of CO2 capture.
Unfortunately, reactor designs which are amenable to flowing solid sorbents present issues with management of those mobile solid sorbent. For example, sorbents of a particle size capable of efficient CO2 adsorption are often easily aerosolized, carried into a flue stream, and progressed further through the reactor system where they cause damage to downstream components and are overall lost. Sorbent particles of sufficient size to not be at risk for being aerosolized are significantly less efficient at sorption per unit mass, which leads to an increase in the mass of sorbent required. Further, sorbent particles themselves are vulnerable in industrial processes as they do not have the structural integrity necessary for prolonged use in reactors. Where the sorbent has low structural integrity and readily breaks down, greater material investment is required and the sorbent becomes less economical to utilize over other competing materials and methods.
Basic Immobilized Amine Sorbents (BIAS or sorbents) and their associated processes are among the most widely studied solid sorbents to mitigate post-combustion carbon dioxide (CO2) emissions. BIAS are organized into three classes (1-3) according to their preparation procedure and amine immobilization mechanisms. Class 1 sorbents are generally prepared by dry or wet impregnation of a support, namely different grades of silica, with a polyamine/hydrophilic solvent (methanol, ethanol, etc.) mixture. Principal polyamines employed are tetraethylenepentamine (TEPA), polyethylenimine (PEI), and generally various linear or branched polyamines that possess different ratios of —NH2 (primary)/—NH (secondary)/—N (tertiary) amine groups that can potentially adsorb CO2. These polyamines are bound to the supports by Si—OH...—NH2 hydrogen bonding and also ionic SiO−...—NH2+/—NH+ interactions. Primary and secondary amines can capture CO2 under dry and wet conditions while tertiary amines primarily capture CO2 only under humid conditions. The manner of amine deposition on the support can be random or structured deposition of the amine onto the support. In addition to silica, other supports may include clays, polymers, activated carbons, zeolites, and others.
Class 2 sorbents are typically prepared by wet impregnation of a mixture of a reactive aminosilane and anhydrous hydrophobic solvent, usually toluene, onto a dry, pre-treated silica support. Strict control of the H2O content within the system is maintained to manipulate the subsequent grafting reaction between the aminosilane and the silica support. The grafted aminosilanes are immobilized to the silica support via covalent Si—O—Si linkages. These Si—O—Si linkages are also responsible for immobilizing the aminosilane within the bulk of the pore via polymerization.
BIAS sorption capacity is typically calculated either on a weight-percent-of-sorbent basis or mmol CO2/g-sorbent basis. For weight percent basis, the weight of adsorbed CO2 is divided by the weight of sorbent and multiplied by 100. For the mmol CO2/g-sorbent basis, the weight of adsorbed CO2 is divided by the molecular weight of CO2 (44 g/g-mole), multiplied by 1,000, and divided by the sorbent weight. The sorption capacity of a pelletized sorbent is best measured by exposing the pellet to a CO2 concentration of ppm level to 100% CO2 at 0 to 120° C. for a period of time until the maximum amount of CO2 is adsorbed by the sorbent, usually less than or equal to 1 hour. Preferential determination of pellet CO2 capture capacity involves placing the pellet in a thermogravimetric analyzer (TGA) or fixed bed reactor and exposing the pellet to 10-15 vol % of flowing CO2 and 0 to 10 wt % H2O with a balance of either air or inert He or N2 at 40-75° C. The CO2 concentration range and adsorption temperature here are either in the range of coal-fired power plant flue gas, or can be achieved with minimal process modification. The pellet is first heated at 100-110° C. for 10-60 min under flowing air or an inert gas to remove any pre-adsorbed water or CO2 (from the environment). To determine the CO2 capture capacity of the pellet in the case of the TGA system, the final weight of the sorbent after CO2 adsorption is subtracted from the initial weight of the sorbent after pre-treatment, and the weight difference is used to calculate the CO2 capture capacity. In the case of the fixed bed, CO2 gas concentration profiles from an effluent measuring device, such as a mass spectrometer, are analyzed and used to calculate the CO2 capture capacity of the pellet.
Advancements in reactor design from batch, fixed-bed systems to continuous circulating fluidized bed, rotating disk, and moving bed systems, and development of a steam-stable sorbent under practical conditions are promising milestones towards commercialization. However, the aforementioned inherent difficulties in the application of such a small particle-size sorbent to industry scale processes remain. For example, BIAS degrades structurally over time as the material is moved from one industrial environment to another. Additionally, the light BIAS can be picked up by and carried into a gas stream, leading to loss of the material and degradation of components downstream. Further, the current amine based sorbent technology utilized in CO2 separation is that the impregnated liquid amines of the BIAS sorbents are vulnerable to leaching from the sorbent pores by condensed steam during practical CO2 adsorption-desorption testing under humidified conditions. The deleterious effect of steam on the CO2 capture of BIAS materials is widely seen in the literature, and was attributed to, in part, amine leaching from the sorbents. Additional difficulties with small particle sorbents include high energy costs to overcome large pressure drop across sorbent beds and failure of, specifically, internal moving parts (valves, conveyors, etc.) by agglomerated or aerosolized particles. Because of these issues, pelletization of immobilized amine sorbents is advantageous for their large scale application.
It would be advantageous to provide a pelletized sorbent for CO2 capture using an amine-based solid sorbent, where the pelletized sorbent is capable of efficient CO2 sorption while maintaining an appropriate mass and structural integrity for use in a post-combustion separation system. Such a pelletized sorbent would achieve acceptable CO2 adsorption while being of an optimized volume and mass for incorporation of large loads of the sorbent into a post-combustion reactor system. Additionally, the pelletized sorbent would more easily provide for integration with existing power or fuel production facilities than current solid sorbents. Thus, through utilization of the pelletized sorbent, increases in CO2 capture capability while minimizing energy and infrastructure requirements is realized.
Accordingly, it is an object of this disclosure to provide a pelletized sorbent for CO2 capture comprising: a Basic Immobilized Amine Sorbent, an inorganic strength additive, and a polymer binder.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.